Magnetic structure



Mamh 10, 1942.` R AREY 2,275,880

MAGNETIC STRUCTURE Filed oct. 5, 1939 Ivetov: Ralph@ Arey, b )Vm/'7fv yHis Att'brney.

Patented Mar. 1942 Ralph G. Arey. Swampscott,

Mau., assignor to General Electric Company, a corporation of New YorkApplication October 5, 1939, No. 298,154 5 Claims. (Cl. 175-21) Thisinvention relates to permanent magnets and concerns particularlypermanent magnets arranged in a. magnetic circuit to provide magnetic uxin an air gap, such for example, as that employed in sound translatingdevices of the dynamic type, and electrical measuring instruments of theDArsonval type.

An object of my invention is to provide an improved permanent magnetstructure in which the leakage liux is reduced to a minimum sothat theavailable magnetic energy of the material may be utilized to itsgreatest elcicncy.

Another object of my invention is to provide an improved magneticcircuit including a permanent magnet of novel construction for producingan intense magnetic lleld in an air gap.

A further object of my invention is to provide a magnetic structure ofthe above character which shall be of simple construction and which maybe manufactured and assembled in an economical manner.

Other objects and advantages of my invention will become apparent as thedescription proceeds.

In accordance with ferred form, in order to obtain avery stronginagnetic ilux with a. minimum weight and bulk of magnetic material, Iemploy a permanent magnet of high coercive force material and utilize acon-` struction for the magnetic circuit whereby the leakage area isreduced to a minimum. I provide a radially magnetized hollow cylindricalpermanent magnet having a sphere-shaped closed end portion with itsinner surface joined icy a cenc tral core member of soit iron or otherrelatively permeable magnetic material. The outer cylindrical andspherical surfaces of the magnet are joined by a magnetic shell or yokewhich serves as a part of the magnetic return circuit. In oneemhodimentvl provide an annular ring of relan tively permeable magneticmaterial with its outer annular surface joined to the inner surface ofthe shell adjacent the open end of the latter, and with its innercircular surface serving as a pole face. A working air gap is formedbetween the central core and the annular pole face within which isestablished an intense magnetic iield. As a modiilcation I may form themagnet from a plurality of rings with the sphere-shaped end portionformed by several individual segments or as a single piece. Or, ifdesired, the sphereshaped end portion may be dispensed with and abar-shaped permanent magnet employed to join the end of the central coreto the outer shell. In this latter construction the annular ring myinvention in its prel which serves as the pole face may be formedintegrally with the outer shell, in which case the opposite end of theshell may be closed by a permeable cap or closure member.

As stili another modiiication, I may employ a plurality of bar-shapedpermanent magnets positioned with their like poles of one polarity inengagement with the central core and their like poles of the oppositepolarity in engagement with the inner surface of the shell.

The novel features which are characteristic of my invention are pointedout in the appended claims. My invention, however, will be understoodmore readily by reference to the following detailed description whenconsidered in connection with the accompanying drawing in which Fig. 1is a cross-sectional view of a magnetic circuit including a permanentmagnet .constructed in accordancewith my invention; Fig. 2 is asectional view taken along the line 2 2 of Fig. l; Figs. 3 and 4 aremodifications of Fig. 1 showing my invention applied to an electricalmeasuring instrument; Figs. 5 to 10 inclusive, are corresponding viewsof various modified forms of magnetic circuits utilizing the principlesof my invention; and Fig. ll represents apparatus which may be employedfor magnetizing the permanent magnet. l

Referringto Figs. l and .2 of the drawing, for the purpose ol"illustrating my invention, I have shown a permanent magnet fieldstructure I0 providing an annular air gap Ii which is adapted to receivea vicratory coil i2 such as the voice coil oi a loudspeaker. In Figs. land 2 the numeral i3 represents e. hollow permanent magnet in the formof a cylinder with e. closed sphereshaped end portion it. Within thecentral opening i5 ci the .magnet is tightly pressed a central coremember te. A. yoke or shell Il,

having its inner surface made to conform to and engage the externalsurface of the magnet I3, provides a low reluctance return path for themagnetic flux set up by the magnet and in conjunction with the core I6serves to concentrate the ux in the `air gap II. To provide a rigidsupport for the core I6 relative to the yoke I1, I employ an annulardisk I 8 of brass or other suitable non-magnetic material having acentral opening I 9, the surface of which is adapted to t in pressedengagement with the core I6. The external annular surface of internalsurface of the shell I1.

I provide a circular end plate 20 having its external cylindricalsurface in rm engagement with the internal surface of the yoke or shellIl,

the disk engages the in the eld to below 100 The end plate is providedwith a concentric opening 2| which cooperates with the outer surface 22of the core to define the annular air gap I I in which it is desired toconcentrate the magnetic flux produced by the magnet I3.

The member constituting the permanent magnet I3 is composed of permanentmagnet material, preferably material having a relatively high coerciveforce, as compared, for example, to the old or usual form of chromesteel, in order that a high degree of magnetization may be maintainedand a compact construction maybe used with the field producing member.I3 magnetized in the direction of its short dimension. While myinvention is not limited to the use of any particular materials, I havefound that highly satisfactory results may be obtained by the employmentof permanent magnet alloys containing iron, nickel, and aluminum as thebasic or essential ingredients, as'described in U. S. Patents 1,947,2'74and 1,968,569, to William E. Ruder, and 2,027,994 to 2,028,000,inclusive, to Tokushichi Mishima.' Or if a sintered magnet formed fromsuch alloys is employed, it may be formed in accordance with thesintering process disclosed in a copending application, Serial No.196,691, led March 18, 1938, Patent No. 2,192,743, to Goodwin H. Howe,entitled Sintered permanent magnet, and assigned to the same assignee asthe present invention. The coercive force of these alloys isapproximately 440 oersteds and they have a residual induction of 'aboutr7500 gausses.

Another material which is highly satisfactory for providing a permanentmagnet of unusually short length in its polarized axis is prepared andmagnetized as follows:

Mix together finely powdered magnetite, ferric oxide, and cobaltic oxidein the proportion of 43.6 per cent of magnetite, 30.1 per cent of ferrieoxide, and 26.3 per cent of cobaltic oxide. Mold the mixture in theshape desired under pressure of from `three to five tons per squareinch. Remove from the mold and heat in an atmosphere of nitrogen or airfor two to three hours at about 1020 C. and allow to cool.` Then reheatto about 520 C. in a special furnace placed in the air gap of adirect-current electro-magnet with la eld of about 3000 h. With the eldon, lower the temperature to about 300 or 320 C. and hold in the fieldwithin this range of temperature for about three quarters of an hour.Then allow to cool C. The material may then be machined or ground toshape if necessary.

The permanent magnet strength of this ma-v terial is several times thatof ordinary permanent magnets, such for example as those made fromchrome steel, for the same size and Shape of magnet and the material isonly about half the weight of such steel. This material prepared aspreviously described has a coercive force between '700 and 1000 oerstedsand it has a residual induction of about 2,200 gausses. Due to this highcoercive force, the sintered oxide permanent magnet can be made veryshort in its polarized axis and is thus particularly suitable whereeconomy in space is an important item.

The magnet I3 is magnetized radially in order to send a flux through theworking air gap I I via the core IB, the yoke Il., and the annular polepiece 20. This arrangement provides a magnet of relatively short lengthin its polarized axis and of large cross-section. The direction ofmagnetization is represented by the arrows.

The central core or pole piece I6, the annular pole piece 20, and theyoke I1 are preferably composed of soft iron or other relatively highpermeability, low coercive force, magnetic material.

Inasmuch as the central core member I5 must carry all of the magnetizingflux it tends to become saturated at a value below that necessary tomagnetize the entire area of the magnetic material I3 at one time sothat it is diicult to magnetize the material to the saturation point.However, for short axial lengths, the core I6 does not saturate and themagnet I3 may be magnetized by simply removing the circular pole piece20 and applying a suitable magnetizing force across the core I6 and theyoke or shell Il.

Figs 3 and 4 represent respectively perspective and cross-sectionalviews of an electrical measuring instrument of the DArsonval typeembodying the principles of my invention. The instrument comprises aninstrument mechanism consisting of a movable element or currentconducting coil 23 mounted with a pair of magnetic eld elements similarto I0 of Fig, 1 and provided With a pointer 24. The magnetic eldcomprises a cylindrical core 25 mounted substantially coaxial with theaxis oi rotation of the element 23, a pair of central pole pieces orflux carrying elements I6 positioned in axial alignment with their polefaces 26 located in spaced opposed relation to form air gaps 21 for themovable element. A permanent magnet I3 is positioned about each of thecores as explained in connection with Fig. 1 and a pair of magneticelements I1, each of which has its internal surface in contact with theexternal'surface of one of the magnets, provides a magnetic ux returnpath.

As indicated clearly in the drawing the relatively permeable members I'Iwhich form the flux return path are joined together in any convenientmanner at 28.

The operation of the device should be clear from the description ofFigs. 1 and 2. It will be understood that the magnets are polarized in adirection normal to their internal and external surfaces and, of course,they will be polarized to produce magnetic flux in the same relativedirection as indicated by the arrows.

In the embodiment illustrated in Figs. 5 and 6, I have provided a modiedarrangement of the permanent magnet I3 which incorporates the desirablefeatures of my invention and which at the same time provides a magnetstructure which may be more readily magnetized. In this case thepermanent magnet I3 is composed of a plurality of plane-faced concentricrings 30 in axial alignment, the rings being positioned with their planesurfaces side by side and each Vring having a central opening 3l foraccommodating the central core I6. The closed end of the magnet may begiven a sphere-shape by means of an annular segment 32 and an endsegment 33. The rings are radially magnetized as indicated by the a1'-rows.

Fig. '7 illustrates a further modification wherein the rings 30 formingthe magnet I3 of Fig. 5 are not continuous as shown in Fig. 6 but areformed by a plurality of curved segments 3A.

Either of the arrangements illustrated in Figs. 6 and l may be employedin composing the magnet I3 of Fig. 5 for use with the magnetic struc-`tures of Figs. 1 and 3. While I have shown the sphere-shaped end portionof the magnet I3 in Fig. 5 formed by the segments 32 and 33, myinlindrical bar-shaped permanent magnet 31,'magnetized in the directionof its axis as indicated by the arrow. With this construction I find itadvantageous to make the end pole piece 33 integral with the cylindricalmagnetic yoke or shell 3S. The non-magnetic disk I8 performs the samefunction as that described in yconnection with Fig. 1, namely, to holdthe core 35 and the yoke 39 in fixed relative relation and ltherebymaintain a precisely dimensioned annular air gap II between the outersurface 22 of the core and the inner surface of the central opening orannular pole face 2|. To complete the assembly, I employ a cap orclosure member 43 of relatively permeable material provided with anannular recess 4I for accommodating the end of the magnet element 3l andhaving an internally threaded annular flange 42. The threads on theinternal side of the flange 42 engage the threads on the yoke or fluxreturn element 39 to hold the parts firmly together.

The method of assembly of the magnetic cirf cuit of Fig. 8 should beobvious. The central pole piece 35 is positioned in the central openingof the pole piece 3B with the annular disk I3 arranged to hold the partsfirmly in place as shown. The rings are placed on the core or pole piece35 in succession, the magnet element 31 is positioned against the end ofthe core with one of its polar surfaces'n engagement with the endsurface 33 and the return iluxpath is completed by the closure member40.

In Figs. 9 and 1G I have illustrated still another modiiication whichembodies several of the features of the arrangement of Fig. 8 butemploys a diiferentform of permanent magnet. The permanent magnet is inthis instance composed of a plurality of bars 43 magnetized to send aradially directed fluxbetween the central pole piece 35 and the yoke 39,as indicated by the arrows. The bars 43 are radially positioned aboutthe central core member 35 as clearly shown in the sectional view ofFig. 10.

I have shown in Fig. 11 by way of example, convenient apparatus whichmay be employed for magnetizing the annular rings 30 and the segments 32and 33 which form the permanent magnet I3 of Fig. 5. The arrangementillustrated comprises an electromagnet 44 having a plurality of polepieces 45 and 46. The pole piece 45 is provided with an annular poleface 41 which engages the outer magnetic yoke or shell II of the magnetstructure I0. The other pole piece 46 of the electromagnet is composedof a highly permeable material and is of sufiicient size to accommodatethe rings 30 and thedisk I8 and of suilicient length to leave an airgapv contact with the inner surfaces of the segments. The rings 3l! anddisk I3 are positioned on the core 46 and the magnetizing operation isready to start.

Upon energization of the electromagnet the 'resulting magnetic Y-iluxset up in the core 46 crosses the air gap 48, passes through the coreI6, the segments 32 and 33 in the directionof the arrows, and the yokeII to the pole 45. ,The segments 32 and 33 are thereby magnetized tosaturation. The ring 30 adjacent the air gap 48 is then moved toward theair gap to the position indicated by thedotted lines so that its outercylindrical surface engages the annular pole face 4'I and the ring isthereby radially magnetized. The ring may then be moved across the airgap and positioned on the core adjacent the annular segment 32. Theother rings are then magnetized in succession. At the end of themagnetizing process, the non-magnetic disk I8 is placed in position asshown in Fig. l and the circular pole piece 2li is then pressed into theshell or yoke I7 as indicated.

In prior devices of this character, where the permanent magnet isarranged in a central portion; e, g., in the same relative position asthe core I5, or where it is arranged in the form of a yoke, much of theuseful flux produced by the magnet is lost because of leakage. With thearrangement of the present invention, the leakage area is reduced to aminimum since the external surface of the magnet is almostcompleteappreciable short-circuiting of the magnet flux.

Magnetic circuits employing permanent magnets so arranged have beenfound to compare Very favorably with electromagnets and the principlesemployed in the present invention as illustrated by the severalembodiments have greatly enlarged the field of usefulness of permanentmagnet devices for numerous applications, many of which will readilyoccur to those skilled in the art. For example, other forms of magnet.and pole pieces may be employed and by a suitable rearrangement of thepole faces to give the desired contact area a holding magnet may bereadily obtained.

In accordance with the provisions of the patent statutes, I havedescribed the principle of operation of my invention together with theapparatus which I now consider to represent the best embodiment thereof,but I desireto have it understood that the apparatus shown is onlyillustrative and that the invention may be carried out by other means. i

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. In amagnetic circuit, a permanent magnet comprising a cylindricalportion having a central axial opening therein and a second portionforming a closure for one end of said opening, each of said portionsbeing polarized in a direction normal to7 the surface defining saidopening, a central core providing a pole piece and posi= tioned in saidopening in engagement with the surface dening said opening, and amagnetic flux return element in engagement with the external surfaces ofsaid portions and cooperating with means including said pole piece toprovide a working air gap.

2. In a. magnetic circuit, a permanent magnet having a cylindricalportion and a sphereshaped end portion and provided with a centrallongitudinal cylindrical opening therein, said magnet being polarized ina direction normal to the wall of said opening, a pole piece having aportion positioned in said opening and in engagement with said wall, anda return ilux path including a second pole piece and a relativelypermeable element engaging the external surface of said magnet, saidsecond pole piece cooperating with said first pole piece to define aworking air gap.

3. A magnetic structure comprising a central core, a cylindrical yokehaving a sphere-shaped end portion and mounted in spaced relation aboutsaid core, a permanent magnet having a shape similar to said yokepositioned in the space between and in contact with said core and saidyoke, said magnet being polarized in adirection normal to said core andsaid yoke, and a magnetic element secured to said yoke -and deiining aworking air gap with said core.

4. In a magnetic circuit, a central pole piece, a permanent magnetpositioned about and in contact with said rpole piece, said permanentmagnet having surfaces of the character formed by the revolution of twosubstantially U-shaped spaced lines and polarized in a direction normalto said spaced lines, a return iiux path comprising a relativelypermeable magnetic element in engagement with the outer surface of saidmagnet,l and an annularpole piece in engagement with said magneticelement and including an annular pole face defining an air gap with saidcentral pole piece.

5. In a, magnetic structure, a permanent magnet arranged and constructedas a substantially closed hollow member and having a single openingbetween the interior and exterior of said member, said magnet beingmagnetized in such la direction that the inner surface of said magnetforms one pole and the outer surface of said magnet forms a second pole,a pole piece engaging substantially all of said inner surface of saidmagnet and having a portion extending outwardly through said opening, asecond pole piece engaging substantially all the outer surface of saidmagnet except a small portion adjacent said opening Which is out ofcontact with said second pole-piece to an extent sufficient to preventappreciable short-circuiting of the magnel; flux, said second pole piecehaving a portion closely adjacent to the extending portion of said rstpole piece to form a working air gap whereby the iiux leakage betweenthe poles is minimized.

RALPH G. AREY.

